Electrolyte for and method of anodically polishing nickel



Patented Oct. 28, 1947 'UNITED STATES PATENT OFFICE ,ELECTROLYTE FOR AND METHOD OF ANODICALLY POLISHING NICKEL Charles L. Faust, Columbus, Ohio, assignor to Battelle Memorial Institute, Columbus, Ohio, a

corporation of Ohio No Drawing. Application April 24, 1942, Serial No. 440,338

concentration. While it is possible to produce polishes with aqueous mixtures containing as much as 50% water, it has been found preferable to keep the water content of the electrolyte bath relatively low, since, in general, baths containing lower proportions of water may be operated at lower current densities, and therefore at a lower cost, to give satisfactory polishes. The relative proportion of the essential acids may be varied from about to 85% each. Unless otherwise specified, all percentages referred to herein will be understood to represent percentages by weight rather than by volume. Also, references to the acids should be understood to mean the respective acids themselves as represented by their formulae, such as H2804 and HsPO4.

Thus nickel is electropolished to a brilliant mirror-like finish at 100 to 130 F. and at a current density of 100 to 250 amperes per square foot in an aqueous bath containing sulfuric acid and 63% orthophosphoric acid within a relatively short time determined by the original physical character of the nickel being polished. However,

in order to maintain continuous bath operation,

it is necessary periodically to remove dissolved nickel from the bath. Before removal of the dissolved nickel, the nickel concentration may in-' crease to as much as 4% (by weight of the bath) even though 1 to 1.5% is the apparent saturation point for a stable nickel solution. The exact action by which nickel supersaturates the solution is unknown. When the bath is heated, nickel in excess of the saturation amount of 1 to 1.5% is precipitated as nickel sulfate. Consequently the bath, on continued use, will precipitate nickel sul- 6 Claims. (01. 204-140) nickel sulfate increase the tank voltage and greatly decrease heating and cooling eiliciency. I

If the bath is operated at a temperature high enough to precipitate nickel sulfate continuously, for instance, at 180 F., then a current density of more than 250 amperes per square foot must be employed as otherwise the nickel surface, although given a brilliant finish, will be pitted. The high current densities which are required to prevent pitting at high temperatures are undesirable from a commercial standpoint.

To overcome these difficulties, the bath can be temporarily reconditioned by being heated at a high temperature so as to precipitate nickel sulfate, the composition of the bath then being adjusted and operation being continued until reconditioning is again necessary. Such reconditioning is not feasible in a production tank, hence separate tanks and attendant auxiliary equipment are needed.

It is therefore an important object of the present invention to provide an electrolyte and a method for electro-polishing nickel capable of being operated continuously at low current densities without need for batch reconditioning, and

without impractical change in composition.

A' further object of the invention is to provide an electrolyte and a method for electropolishing fate on the hot surfaces of heating coils and on v the cathodes (which are heated by electric current flowing therethrough). Such deposits of nickel capable of continuous operation without a progressive rise in operating voltage.

Other and further objects and features of the present invention will become apparentfrom the following description and appended claims.

I have found that the addition of trivalent aluminum, or of trivalent aluminum and trivalent chromium, to the mixed sulfuric acid-phosphoric acid baths referred to hereinabove (such as the bath containing 15% sulfuric acid and 63% phosphoric acid) yields baths in which nickel can be electro-polished at temperatures around 175 F. or higher at current densities from 125 to 250 amperes per square foot, dissolved nickel in excess of the saturation amount being precipitated continuously as a sulfate. Tank voltages and current densities are essentially the same as for the low temperature bath containing 15% sulfuric acid and 63% orthophosphoric acid, but treating times are shorter owing to a higher anode efliciency of the baths containing trivalent aluminum and trivalent chromium operated at a high temperature.

Thus, although trivalent aluminum and trivalent chromium are not essential for the production of a brilliant mirror-like electropolish on nickel in a bath containing mixed sulfuric and orthophosphoric acids, yet these added ingredients serve to make possible such electropolishing under conditions more favorable for continuous plant operation. Separate conditioning tanks and treatments are not required. Occasional filtration is all that is required to remove the precipitated nickel sulfate.

The addition of trivalent aluminum, or of trivalent aluminum and trivalent chromium, has a like favorable effect on all the mixed sulfuric acid-phosphoric baths referred to hereinabove, as evidenced by the fact that the addition of trivalent aluminum and trivalent chromium to a bath 7 containing sulfuric acid and 63% orthophosphoric acid diluted to about 27 water serves to make continuous electropolishing possible at 115 to 130 F. The addition of trivalent aluminum, or trivalent aluminum and trivalent chromium, eliminate pitting and permits electropolishing at temperatures higher and at current densities lower than those otherwise required for electropolishing. Thus commercial operating temperatures can be selected in the range of 100 to 210 F. in baths which eliminate the need for special reconditioning treatment. Specific operating temperature ranges are broad enough forcommercial control and selected according to bath composition. Greater additions must be made in proportion to the departure from the operating conditions under which the corresponding bath containing a straight sulfuric acid-phosphoric acid mixture is operated. However, excess dissolved nickel is still precipitated as a salt which must be removed from the polishing tank,

Illustrative examples of baths containing trivalent aluminum and trivalent chromium are given hereinbelow.

A bath containing. 15% sulfuric acid and 63% orthophosphoric acid together with 0.5 to 2.6% trivalent aluminum will yield a smooth brilliant nickel electropolish at 175 F. and a current density of from 125 to 250 amperes per square foot. The preferred bath contains additionally from 0.5 to 1.4 trivalent chromium, the exact amounts value corresponding to the bath saturated with from 0.5 to 1% nickel. Water must be added at intervals or continuously to maintain the composition of the bath.

The addition of trivalent aluminum, or of triacid mixtures give pitted polishes at 175 F., they are all operative to produce satisfactory polishes at higher temperatures and lower current densities on addition of trivalent aluminum or of trivalent aluminum and trivalent chromium the relationship being HzSO4-HaPO4 ratio and cell voltage remaining the same.

140 F. 175 F. Ratio cell voltage cell voltage cell voltage cell voltage HBPO' at 125 amperes at 250emperes at 125 amperes at 250 amperes in bath per square per square per square per square foot ioot foot foot The percentage compositions and operating temperatures of specific baths according to the present invention are tabulated as follows:

Bath No. 1 Bath No. 2 Bath No. 3 Bath No. 4

HaSOl l4. 6 14. 9 l3 l0 HsPO| 60. 6 62.3 56 i 62 l. 3 0. 3 l. 4 None Water- Balance Balance Balance Balance Operating tempereture.. F" 176 116 to 130 115 to 130 175 The baths of the present invention are operative for the electropolishing of nickel containing alloying ingredients in amounts such as will not materially afiect the behavior of the nickel for electrolytic treatment purposes. The principles of the invention include broadly electropolishing valent aluminum and trivalent chromium, is also effective to improve the operation of other baths for electropolishing nickel containing mixed sulfuric and phosphoric acids. Very good results are secured by adding 1% trivalent aluminum and 1% trivalent chromium to a bath containing 20% sulfuric acid and 67% orthophosphoric acid. However, the bath containing 15%-sul furic acid and 63% orthophosphoric acid produces the best electropolishi'ng results and possesses the most favorable voltage characteristics, as shown by the following table. Although the tabulated straight phosphoric acid-sulfuric nickel in baths containing'trivalent aluminum, to which trivalent chromium may be added, when relatively low current densities are satisfactory at temperatures upwards of or F. The addition of trivalent aluminum and chromium is of particular interest in the electropolishing of nickel'at high temperatures, as when electropclishing is carried out at a rapid rate, since such additions lower the current density requirements below those of conventional baths.

This application is a continuation-in-part of my application Serial No. 241,975 filed November 23, 1938, entitled Electrolyte for the polishing of metal surfaces, and now issued as United States Patent No. 2,334,699.

Many details in composition and procedure may be varied within a wide range without departing from the principles of this invention, and it is therefore not my intention to limit the patent granted on this invention otherwise than necessitated by the scope of the appended claims.

I claim as my invention:

' .5. 1. An electrolyte for use in the anodic polishing or nickel comprising from to 85% by weight orthophosphoric acid, the combined content of these acids being at least 50% but not over 90% by weight of said solution, 0.5 to 2.6% by weight trivalent aluminum and 0.5 to 1.4% by weight trivalentchromium, the balance comprising essentially water.

2. The method of electrolytically polishing nickel which comprises making the nickel the anode in an electrolyte comprising from 5 to 85% by weight of sulfuric acid and from 5 to 8.5% by weight of orthophosphoric acid, the combined content of these acids being at least 50% but not over 90% by weight of said solution, 0.5 to 2.6% by weight trivalent aluminum-and 0.5 to 1.4% by weight trivalent chromium, the balance comprising essentially water, and. passing a current through said electrolyte of a sufllcient density and of sulfuric acid and from 5 to 85% by weight of V continuously as a'sulfate, and filtering off said for a sufilc ient period of time to effect a polish v on said nickel.

3. The method of continuously electrolytically olishing nickel which comprises making the nickel the anode in an electrolyte comprising from 5 to 85% by weight of sulfuric acid and from 5 to 85% by weight of orthophosphoric acid," the combined content of these acids being at least 50% but not over 90% by weight of said solution, and 0.5 to 2.6% by weight trivalent aluminum, the balance comprising essentially water, and passing a current thro'ughsaid electrolyte at a temperature between 100 and 210 F. and at a current density of from 125 to 250 amperes per square foot and for a suihcient period or time to effect a polish on said nickel, filtering of! nickel sulfate precipitated in said electrolyte and continuing the passing of current through said electrolyte to effect further polishing of said nickel.

4. The 'method of electrolytically polishing nickel which comprises making the nickel the anode m an electrolyte comprising from 5 to 85% by weight of sulfuric acid and from 5 to 85% by weight of'orthophosphoric acid, the combined content, of these acids being at least 50% but not over 90% by weight of said solution, 0.5 to.2.8% by weight trivalent aluminum and 0.5 to 1.4%

by weight trivalent chromium, the balance comprising essentiall water; and passing a current through said electrolyte at a temperature between 100 and 210 F. and at a current density of from 125 to 250 amperes per square foot and for a sufilcient period of time to effect a polish on saidnickel, and filtering off nickel sulfate precipitated in said electrolyte. 5. The method of continuously electropolishin nickel which comprises making the nickel the precipitate from said electrolyte, said electrolyte being maintained at a temperature below 180 F. but not below about 100 F. during the foregoing operations.

6. The method of continuously electro-polishing nickel which comprises making the nickel the anode in an electrolyte comprising from 5 to by weight of sulfuric acid and from 5 to 85% by weight of orthophosphoric acid, the combined content of these two acids being at least 50% but not over by weight of said solution, and 0.5 to 2.6% by weight trivalent aluminum, the balance being essentially water, passing an electric current at a current density of from to 250 amperes per square foot through said electrolyte for a sufiicient period of time to effect a polish on said nickel, nickel dissolving in excess of the saturation amount being precipitated continuously as a sulfate, and filtering ofl said precipitate from said electrolyte, said electrolyte being maintained at a temperature below F. but not below about 100 F. during the foregoing operations,

CHARLES L. FAUST.

REFERENCES crrEn" The followingreferences are of record in the O'I'HER REFERENCES Metals and Alloys," vol. 13, Apr. 1941, pages 424 through 430; publication by Meyer et a1. 

